A number of insects and their larvae in the soil are known to cause serious damage to agricultural and horticultural crops. These insects and their larvae include root flies such as the cabbage rootfly (Hylemyia brassicae) and wireworms (Agriotes, the larvae of the click beetle). For example, wireworms occur in practically all economically important crops e.g. wheat, sugar beet, carrots, potato and maize. Wireworms are a serious threat to world food production.
Insects and their larvae in the soil have been effectively controlled with halogenated hydrocarbons such as aldrin, dieldrin, heptachlor, D.D.T. and B.H.C. (benzene hexachloride). Unfortunately these compounds leave heavy and extremely persistent residues in the soil, amounting typically to 2 ppm from a single application of the compounds. The loss of these residues from the soil by leaching is very slow, and they are not fully broken down in the soil for many years. Furthermore, halogenated hydrocarbons tend to be concentrated and stored indefinitely in the lipid fraction of animal tissues, and species occupying the top positions of food chains, such as birds or prey and man, receive appreciable doses of these materials from normal agricultural use. Although previously considered to be valuable pesticides, the majority of these materials are now either banned completely in developed countries or restricted to a few specific uses.
Many alternatives have been found to the halogenated hydrocarbons for control of insects on plant foliage (topical application); for example, many organophosphorus derivatives have been found to be equally effective for this purpose. For use in the soil, on the contrary, it has been most difficult to find acceptable replacements for such compounds as aldrin and heptachlor.
Soil often has a pH quite widely different from neutrality; it also contains materials having catalytic activity and adsorptive properties, and it is infected with micro-organisms which have a wide range of biological capabilities. The combined effect of these factors is to degrade or inactivate the majority of insecticides applied to the soil quite rapidly, even if they are not leached out by railfall.
Most organophosphorus insecticides are either not active in the soil, or have an uneconomically short period of effectiveness. Some organophosphorus compounds which do show activity in the soil, such as phorate, are systemic and leave substantial crop residues. Wireworms have proved particularly resistant to most organophosphorus compounds, and without halogenated hydrocarbons control of this pest has often been barely adequate.
There is a long felt need for a compound which has potent and sustained insecticidal activity in the soil, but which does not contaminate the environment with objectionable toxic residues.